The long term objective of this research is to characterize two membrane associated mitogens which stimulate the proliferation of the glial cells responsible for the production of the myelin sheath (Schwann cells and oligodendrocytes). One mitogen is found on the surface membrane of axons and is a potent mitogen for glial cells early in development when glial cells initially contact axons. The other mitogen is an integral part of the myelin membrane and is released following phagocytosis of myelin by macrophages after neural injury. Using neuritic membrane fractions of cultured neurons we will investigate the possibility that there are two classes of neurons we will investigate the possibility that there are two classes of neuronal mitogens: an "early" form which is not dependent on serum for its mitogenicity and a "late" form which is serum dependent and is associated with the axolemma-enriched fractions. The ability of axolemma-enriched fractions to upregulate the expression of glial receptors for soluble growth factors such as platelet-derived growth factor and fibroblast growth factor will be investigated. We will further investigate the potential identity of the fibroblast growth factor associated with axolemma-enriched fractions as the axonal mitogen for Schwann cells and oligodendrocytes. The role of glial cell will be examined. A battery of peptides which overlap the entire sequence of myelin basic protein and antibodies to those peptides will be used to establish the region of myelin basic protein which is released when myelin is catabolized by macrophages and releases a soluble Schwann cell mitogen. Oligodendroglial and Schwann cell lines will be created by transfection of primary cells with a plasmid containing the coding region for large T antigen to cause the cells to divide; stable antisense constructs to large T antigen will then be used to stop proliferation so that these cells can be used for proliferation assays. This investigation is important since myelinating cells must proliferate before they myelinate either early in development or after neural injury. An understanding of the molecular aspects of such proliferation will enable us to design better strategies for remyelination and will provide a baseline from which to evaluate the uncontrolled proliferation characteristic of glial cell tumors.